1. Field of the Invention
The subject matter disclosed herein relates generally to a gas measuring device based on infrared absorption and, specifically, to a simplified filter and beam splitter arrangement used to divide a source radiation into a measuring and a reference part or to combine the radiation from a measuring and a reference source into one radiation path. The measuring device is typically a single path non-dispersive infrared analyzer using narrow-band transmission filters to enable selectivity for a specific gas and a non-absorbing reference band. The device may have either one source and two detectors or two sources and one detector.
2. Description of Related Art
Beam splitters are used in optics for the purpose of dividing one beam into two parts or combining two beams into a single beam. Wavelength region or distribution and intensity ratio between the two separate beam portions depend upon the specific properties of the beam splitter. The most typical beam splitter is a thin plate of glass or plastic with one surface coated to be a semi-reflective mirror. One portion of the beam is transmitted through the beam splitter and the other portion is reflected, typically by 90 degrees if the angle of incidence is 45 degrees. Possible absorption in the beam splitter materials is here ignored and is usually very low for the designed wavelength regions. Further, a thin semi-reflective membrane, a pellicle, is a possible beam splitter solution but it may not be robust enough in many cases and it can be sensitive to temperature fluctuations, and its reliable fastening is also a problem. The coating is in most cases virtually independent of wavelength but it may also divide the spectral region into an upper and a lower part. Such a beam splitter is called dichroic and has the advantage that both the transmitted and the reflected radiation suffer little loss. However, the measuring and reference wavelength regions must be well separated for this kind of beam splitter to work with satisfaction. In such known beam splitters the dichroic beam splitter is made of a material which preferably deflects rays having a radiation wavelength in the range of 4.3 microns and alternately transmits wavelengths in the range of 3.7 microns.
It is known an apparatus for analyzing the breathing gases of a person for alcohol, the analysis being carried out on the basis of infrared radiation absorption properties. For this purpose the apparatus comprises a radiation source for providing an infrared radiation beam, a measuring chamber for selectively receiving the breathing gages of a person or ambient air, whereupon the radiation from the radiation source passes through the contents of the measuring chamber, a first detector for receiving radiation that has passed through the measuring chamber, and a first optical interference bandpass filter interposed in front of the detector along a path of the radiation beam. The first filter has a radiation wavelength transmission band for transmitting radiation of the transmission band wavelength through the filter to the first detector, while the reflected radiation has wavelengths outside the radiation wavelength transmission band. The first filter is here movable to a plurality of positions inclined with respect to the radiation beam for altering the radiation wavelength transmission band of the first filter. There is also a second detector positioned to receive radiation reflected from the surface of the first filter when it is moved to a given one of the plurality of positions. Means coupled to the first detector analyze the breathing gases for alcohol on the basis of radiation received by the first detector when it is in a plurality of positions. Comparison means coupled to the first and second detectors establish a relationship between the transmitted radiation received by the first detector and the reflected radiation received by the second detector when ambient air is in the measuring chamber for determining the suitability of ambient air as a reference gas mixture.
It is known a non-dispersive infrared measuring arrangement, in which the beam splitter may consist of two segments positioned on top of each other in a direction transversal to the beam direction of the radiation, which beam splitter segments are designed and manufactured to have different beam splitting ratios. For the second independent single path analyzer channel, the characteristics of the first beam splitter segment, i.e. the splitting ratio and the cross-over wavelength, are selected so that neither an additional measurement filter nor an additional reference filter is needed, but the beam splitter acts simultaneously as the actual beam splitter and as the measurement filter and as the reference filter. This is possible when the beam splitter has the bandpass characteristics corresponding to the absorption peak of the measured gas component, and the gas component does not have any other substantial absorption peak in the wavelength range, where the detector has sensitivity and/or the chamber windows have transmittance and/or the radiation source has emission. The other first independent single path analyzer channel is provided with an optical measurement filter, having a narrow passband, but no additional reference filter, but an additional analyzing filter in front of the measuring detector partial channel. This kind of arrangement is said to be useful when the beam splitter segment has a relatively narrow wavelength range, which is reflected to have a proper wavelength range for the reference partial channel, whereupon the transmitted wavelength range is in many cases too wide necessitating the measurement filter. Accordingly, both independent single path analyzer channels have measuring detectors positioned to receive the transmitting portion of the radiation beam and reference detectors positioned to receive the reflected portions of the radiation beam, while in the first analyzer channel there is a reference filter between the beam splitter and the reference detector in the reflected beam portion, which case means quite expensive construction, and in the second analyzer channel there is no reference filter in the reflected beam portion at all, in which case the reference detector system receives radiation from very wide wavelength range often causing problems or errors. The publication also discloses the electronic processing units for the two single path analyzer channels, whereupon the measurement partial channel and the corresponding reference partial channel of both analyzer channels is connected to one of the electronic processing units to provide the measurement signals and the reference signals thereto.
The beam splitters described above are called physical beam splitters because the complete beam aperture is available in both the transmitted and the reflected part as opposed to the so called geometrical beam splitters, where the radiation is mechanically divided into transmitting and reflecting parts. The beam splitters in this invention are physical beam splitters. In many cases the detectors must measure over a very narrow spectral wavelength range to be really selective for a specific gas. Using beam splitters as described above still necessitates in most cases an additional narrow bandpass filter, which is different from the beam splitter, in front of the measuring detectors. Then it is normally needed at least three optical components, one for the beam splitter and two further narrow-band filters, one for measuring partial channel and another for the reference partial channel, all three with different specifications, i.e. different wavelength ranges for transmittance and reflectance. Three separately designed filters with small wavelength tolerances are required to avoid e.g. an uncontrolled cross-talk. Dielectric filters for the infrared region are expensive so a simplified solution would be beneficial. Dielectric filters are, as generally known, interference filters, which are multilayer filters, the layers made of nonconductive material like some oxides, some fluorides, some sulfides, etc. the materials having at least two different refractive indices and arranged to have proper very small thicknesses in specified order, but not metallic layers, whereupon wanted edge, lowpass, highpass, bandpass, notch etc. properties are attained. Of course the wanted wavelength range shall be taken account when selecting materials and designing the construction.
A conventional single path infrared (IR) gas analyzer construction intended for measurements of a single gas using a reference sensor and a measuring is shown in FIG. 5. In this kind of detection assembly according to the prior there is an infrared source 1 with optional collimating or focusing optics 2 which directs the radiation as a radiation beam 11 into a measuring chamber 3 with input and output windows 4. Inside the measuring chamber is the gas 10 to be measured. If the chamber 3 is in a mainstream measuring adapter the gas flow would be perpendicular to the figure plane, otherwise the chamber could have input and output connections for the gas. The gas inputs and outputs are not shown in the figure. After the measuring chamber 3 the radiation enters a beam splitter 5 where the radiation is divided into a transmitted part 12 and a reflected part 13. The beam splitter is often a semi-reflective surface with minor wavelength dependence. Any wavelength would then split into a two roughly identical parts, a transmitted and a reflected one. An example of this is a partly metalized surface or a plate made of silicon or germanium or any other material, with high refractive index. Obviously, this arrangement wastes signal. If the measurement wavelength and the reference wavelength are clearly in separate regions a preferable beam splitter construction would be an interference filter transmitting an upper part of the spectrum and reflecting a lower part of it, or vice versa. Such an arrangement is called dichroic and has the advantage that both transmitted and reflected wavelengths are effectively transferred. Another advantage is that the interference filter has very little absorption in wavelength regions intended for use. The transmitted radiation part 12, which is normally the measuring radiation, is transferred to a measuring detector 6 via a narrow-band filter 8. This filter makes the analyzer selective to a specific gas. e.g. carbon dioxide, which case the transmission band must coincide with the absorption peak of carbon dioxide. The reflected part of the radiation 13 is directed through a filter 9 to a reference detector 7, which serves as a non-absorbing reference to the measuring part. The filter 9 is normally of narrow-band type and the center wavelength is chosen so that the gas to be measured has minimal influence on the signal collected from the reference detector 7. A position close to the measuring wavelength is often advantageous since disturbing phenomena like water absorption and scattering, absorption from other gases; or thermal behavior of the filters are then as similar as possible. However it is very difficult to design two filters with transmission bands very close to each other without demanding requirements for wavelength tolerance. This affects the expenses of the construction. Therefore, this latter design is seldom used and instead the former design, in which the measuring and reference wavelength regions are clearly separated, is normally used.